For the problem of performance gap between individual cells in retired lithium batteries after group use, which affects the usable capacity of battery pack, a grouping bi-directional equalization method based on variable domain fuzzy control is proposed. Equalization circuits based on single inductor and LC oscillation circuit are respectively used for inter-cell and inter-cell group, to achieve inter-cell equalization and inter-cell group equalization; Variable domain fuzzy control strategy is used to determine the reasonable range of operating current according to State of Health (SOH) of the battery, combined with the relationship between the capacity decay coefficient and the average operational range of State of Charge (SOC); the equalization current is dynamically adjusted according to its mathematical relationship with operating current. To verify the effectiveness of this equalization method, an experimental platform was built and verification simulations were performed. The result of experiments shows, the equalization speed is increased by 25%, comparing to fixed equalization current control strategy; the capacity decay is reduced by 6% and the service life is extended after experiments of 1200 charge-discharge cycles, comparing to traditional fuzzy equalization strategy.

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